This is the ongoing dilemma with a manufacturer trying to innovate - do they not change anything to keep previous customers "happy" knowing that they are on the latest kit or do the try to make the best possible board they can? I suggest the later, otherwise we would be still sailing on Windsurfer One-designs (yes - these are still fun)!
By definition a raceboard fleet isn't a one design fleet even if it contains similar boards/rigs, so where Can 102 has indicated that a "de facto" Phantom/Severne one-design fleet has developed and is healthy in his area this should not prohibit the introduction of new kit. I think the only one-design raceboard still being sold is the Mistral One Design, (and soon the Phantom 295).
In our area (Australia) our raceboard fleets consist mainly of old raceboards with a few new ones thrown in, so having manufacturers being active in producing new raceboards is extremely important to ensure the class doesn't die when the old boards finally expire.
It is also interesting to note that in our area the new designs from Starboard & Exocet have not been much, if any, faster than the old designs, but I am looking forward to the time when a new design blows the socks off the old gear - maybe the new Phantom 377 is this board? At the end of the day the best sailor will always win, almost irrespective of the board they are on - in our area anyway.
I will be looking at getting a Phantom 377 asap - the design looks very interesting ( and my existing AHD is in pieces at the moment!